The human spine consists of segments known as vertebrae linked by intervertebral disks and held together by ligaments. There are 24 movable vertebrae—7 cervical (neck) vertebrae, 12 thoracic (chest) vertebrae, and 5 lumbar (back) vertebrae. Each vertebra has a somewhat cylindrical bony body (centrum), a number of winglike projections (processes), and a bony arch. The arches are positioned so that the space they enclose forms the vertebral canal. The vertebral canal houses and protects the spinal cord, and within it the spinal fluid circulates. Ligaments and muscles are attached to various projections of the vertebrae. The bodies of the vertebrae form the supporting column of the skeleton. Fused vertebra make up the sacrum and coccyx, the very bottom of the vertebral column.
The spine is subject to abnormal curvature, injury, infections, tumor formation, arthritic disorders, and puncture or slippage of the cartilage disks. Modern spine surgery often involves the use of spinal stabilization/fixation procedures such as a vertebral body fusion procedure to correct or treat various acute or chronic spine disorders and/or to support the spine. These procedures may utilize a variety of spinal implants to help stabilize the spine, correct deformities of the spine such as spondylolisthesis or pseudarthrosis, provide rigid support for an effected region of the spine, facilitate fusion, or treat spinal fractures. For example, anterior lumbar interbody fusion (ALIF) is a surgical technique that utilizes interbody implants to treat a variety of spinal disorders, including disc degeneration.
An intervertebral disc may be subject to degeneration caused by trauma, disease, and/or aging. An intervertebral disc that becomes degenerated may have to be partially or fully removed from a spinal column. Partial or full removal of an intervertebral disc may destabilize the spinal column, resulting in subsidence or deformation of vertebrae. Moreover, destabilization of a spinal column may result in alteration of a natural separation distance between adjacent vertebrae. As one skilled in the art can appreciate, excessive pressure applied to the nerves may cause pain and/or nerve damage. Maintaining the natural separation between vertebrae can help to prevent pressure from being applied to nerves that pass between vertebral bodies.
Through an ALIF procedure, a spinal implant may be inserted within a space created by the removal or partial removal of an intervertebral disc between adjacent vertebrae to maintain the height of the spine and restore stability to the spine. Such a spinal implant may be a fusion device that is designed to fuse with adjacent vertebrae through intervertebral bone growth. An interbody cage is an example of a fusion device. Some interbody cages of different designs have been developed to provide mechanical support to the segment being fused with biocompatible implant material and to allow the use of autogenous bone to promote fusion. Other types of spinal implant may also be used. For example, a bone graft may be inserted into an intervertebral disc space during a spinal fixation procedure using an anterior, lateral, or posterior spinal approach. Such a bone graft may be machined to different shapes, contours, sizes, and heights. A preoperative planner can aid in determining the size of the adjacent intervertebral discs and allow the implant to be firmly seated with a secure fit between the endplates. The surface of the bone graft may contain a saw-tooth pattern on the superior and inferior surfaces to minimize migration after implantation.
An anterior spinal approach may be a preferred method for some spinal implant procedures. An anterior spinal approach may require less bone removal and muscle distraction than a posterior spinal approach. In addition, an anterior spinal approach may involve less risk of nerve damage than a posterior spinal approach.
During an anterior spinal approach, a surgical opening may be made in the abdomen of a patient. This opening may extend from the abdomen to an anterior surface of the spine. For some patients, the opening may be ten or more inches in depth. The opening, sometimes referred to as the work site, needs to be large enough to accommodate instrumentation for inserting a spinal implant within a disc space. A discectomy may be performed to remove all or most of a defective or damaged intervertebral disc. The discectomy creates a disc space for a spinal implant. The amount of removed disc material may correspond to the size and type of a spinal implant to be inserted.
Once the work site has been prepared for device insertion, several surgical techniques can be used to distract, size, and insert the spinal implant, depending upon the patient's local anatomy, the pathology, and the surgeon's preference. One method for inserting a spinal implant within a disc space may include distracting the vertebrae with a distraction device to spread open the collapsed disc space and form a disc space that is slightly larger than a height of the implant to be inserted into the disc space. The implant is then inserted into the disc space using an implant holder or an implant insertion device. After the spinal implant is inserted, the distraction device may then be removed.
Another method involves the use of interbody spacers or trials. For example, surgical personnel may distract the vertebrae to obtain the maximum implant height using a distractor and insert a spacer into the disc space to ensure accurate sizing of the spinal implant. Fluoroscopy and tactile judgment can assist in confirming the fit and placement of the spacer. If the spacer is either too loose or too tight, the next size may be used to achieve the desired secure fit. Once the correct sizing is obtained, the spacer can be removed and the spinal implant can be introduced in the correct orientation into the disc space using an implant holder. After the spinal implant is inserted, the distractor may then be removed.